1. Field of the Invention
The present invention relates to piezoelectric bulk wave filters for use as, for example, a band-pass filter. More particularly, the present invention relates to a longitudinally coupled multi-mode piezoelectric bulk wave filter using the piezoelectric longitudinal effect and using coupling between modes having different orders, and relates to an electronic component including such a piezoelectric bulk wave filter.
2. Description of the Related Art
Various piezoelectric bulk wave filters have been used as a band-pass filter. For frequency ranges of several megahertz to several tens of megahertz bands, double-mode piezoelectric bulk wave filters are primarily used because they are easily miniaturized and inexpensive.
This type of double-mode piezoelectric bulk wave filter is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 5-327401.
FIG. 15 shows the cross sectional view of a known double-mode piezoelectric bulk wave filter using the thickness longitudinal vibration mode.
A piezoelectric bulk wave filter 201 includes a piezoelectric plate 202 undergoing a polarizing process in the thickness direction. On the top surface of the piezoelectric plate 202, a pair of excitation electrodes 203 and 204 is provided. On the bottom surface, a common excitation electrode (ground electrode) 205 is disposed so as to face the excitation electrodes 203 and 204 with the excitation plate 202 sandwiched therebetween.
To operate the piezoelectric bulk wave filter 201, an input signal is applied between the excitation electrodes 203 and 205 on the top surface. This causes the piezoelectric plate 202 to produce the symmetric mode shown in FIG. 16A and the anti-symmetric mode shown in FIG. 16B. These modes are combined to define a filter passband. The output is obtained from between the excitation electrode 204 and the ground electrode 205.
In place of the thickness longitudinal vibration mode, there is also known a double-mode piezoelectric bulk wave filter using the thickness shear mode obtained by applying the polarizing process to the piezoelectric plate 202 in the direction parallel to the top surface of the piezoelectric plate 202.
In the known piezoelectric bulk wave filter 201, the strength of the coupling between the symmetric mode and the anti-symmetric mode depends on the distance between the excitation electrodes 203 and 204. The distance determines the frequency difference between the frequency of the symmetric mode and the frequency of the anti-symmetric mode. This determines the passband.
Therefore, in order to obtain a broadband filter, the distance between the excitation electrodes 203 and 204 must be narrowed to increase the degree of coupling between both modes and to increase the frequency difference between the frequencies of both modes.
The excitation electrodes 203 and 204 are normally generated by applying conductive paste by a screen-printing method. There is a limit to how narrow the distance between the excitation electrodes 203 and 204 can be using the screen-printing method. Photolithography enables the distance between the excitation electrodes 203 and 204 to be even more narrowed. Applying photolithography is, however, expensive.
Furthermore, even though the distance between the excitation electrodes 203 and 204 can be narrowed, the capacitance between the input and output is increased and the attenuation is decreased.
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally coupled multi-mode piezoelectric bulk wave filter which achieves widening of the passband, increasing out-of-band attenuation, and being manufactured inexpensively.
According to a first preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter using the piezoelectric longitudinal effect includes at least one laminated piezoelectric body having at least four excitation electrodes disposed substantially parallel and spaced from one another at a distance D, and a plurality of piezoelectric layers, each disposed between two adjacent electrodes of the excitation electrodes, the plurality of piezoelectric layers being polarized in a direction that is substantially perpendicular or substantially parallel to the excitation electrodes, wherein the cross section of the at least one laminated piezoelectric body in the direction substantially perpendicular to the laminated direction of the at least one laminated piezoelectric body is a rectangle having a thickness T, a piezoelectric portion having a thickness that is greater than the thickness dimension T and aligned with the at least one laminated piezoelectric body, an input electrode, an output electrode, a ground electrode, the excitation electrodes including a first group of excitation electrodes electrically connected to the input electrode, a second group of excitation electrodes electrically connected to the output electrode, and a third group of excitation electrodes electrically connected to the ground electrode, the first group and the third group being alternately disposed in the laminated direction, and the second group and the third group being alternately disposed in the laminated direction. In the longitudinally coupled multi-mode piezoelectric bulk wave filter, when an input signal is applied between the input electrode and the ground electrode, mode vibrations having different orders are generated and combined into an output signal between the output electrode and the ground electrode, and the ratio of the thickness T to the distance D is equal to or less than about 0.9.
In the longitudinally coupled multi-mode piezoelectric bulk wave filter, the input electrode, the output electrode, and the ground electrode may be disposed on the piezoelectric portion. This enables the piezoelectric portion aligned with the laminated piezoelectric body to have the input electrode, the output electrode, and the ground electrode easily formed and to be easily electrically connected to the outside even though the laminated piezoelectric body is small.
The longitudinally coupled multi-mode piezoelectric bulk wave filter may further include a piezoelectric substrate for providing the at least one laminated piezoelectric element and piezoelectric portion thereon. In the longitudinally coupled multi-mode piezoelectric bulk wave filter, a plurality of grooves are disposed on one side of the piezoelectric substrate, and a portion of the piezoelectric substrate between each pair of adjacent grooves of the plurality of grooves defines the laminated piezoelectric element and a remaining piezoelectric substrate portion defines the piezoelectric element. In this case, after the piezoelectric substrate is prepared, a plurality of grooves is formed on one surface of the piezoelectric substrate. This achieves easy formation of the laminated piezoelectric body and the piezoelectric portion.
According to another preferred embodiment of the present invention, a groove is disposed on one surface of a piezoelectric substrate for providing the at least one laminated piezoelectric element and the piezoelectric portion thereon, a portion between the bottom of the groove and the other principal surface of the piezoelectric substrate defines the laminated piezoelectric element, and a piezoelectric substrate portion on both sides of the groove defines the piezoelectric portion.
According to another preferred embodiment of the present invention, a plurality of laminated piezoelectric elements are provided on the piezoelectric substrate.
According to another preferred embodiment of the present invention, the mode vibrations having different orders are harmonics waves in the longitudinal vibration mode using the piezoelectric longitudinal effect.
According to another preferred embodiment of the present invention, the mode vibrations having different orders are harmonics waves in the thickness longitudinal vibration mode using the piezoelectric longitudinal effect.
According to yet another preferred embodiment of the present invention, an electronic component includes a casing substrate, a longitudinally coupled multi-mode piezoelectric bulk wave filter according to the above-described first preferred embodiment of the present invention mounted on the casing substrate, and a cap material fixed to the casing substrate so as to cover the longitudinally coupled multi-mode piezoelectric bulk wave filter. That is, the longitudinally coupled multi-mode piezoelectric bulk wave filter according to preferred embodiments of the present invention can be provided as an electronic component using a package including a casing substrate and a cap material.
According to a third preferred embodiment of the present invention, an electronic component includes a casing substrate, a reflecting layer disposed on the casing substrate, and a longitudinally coupled multi-mode piezoelectric bulk wave filter according to the above-described first preferred embodiment of the present invention fixed to the casing substrate via the reflecting layer, wherein the acoustic impedance Z2 of the reflecting layer is less than the acoustic impedance Z1 of a material forming a laminated piezoelectric portion of the longitudinally coupled multi-mode piezoelectric bulk wave filter and less than the acoustic impedance Z3 of the casing substrate.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.